The present invention relates to electronic energy consumption metering and, more particularly, to electronic metering of electric energy consumption in which the direction of integration is periodically reversed in order to cancel out offset errors.
Metering of electric energy consumption employs measurements of electric current and voltage fed to a load and multiplication of the measured quantities to determine the instantaneous power usage. The multiplied values are integrated over time in a register to record the energy consumed. Electromechanical watthour or kilowatthour meters conventionally employ a conductive disk rotated as the rotor of a small induction motor by the interaction of flux fields from both a voltage stator connected across the energy supply and a current stator connected in series with the load. The rotations of the disk are integrated in a geared mechanism for recording the energy used.
Electro-mechanical watthour meters require precision manufacture which limits the amount by which their cost can be reduced. In addition, electro-mechanical devices are inherently less reliable than devices having no moving parts.
In my prior U.S. Pat. Nos. 3,955,138; 4,066,960; 4,485,343 and 4,495,463, (the disclosures of which are herein incorporated by reference), I disclose techniques for electronic energy consumption metering which employ time-division or amplitude-markspace modulation for multiplying an analog variable related to instantaneous current by an analog variable related to instantaneous voltage. The multiplied resulting signal contains a DC component, representing the desired power measurement, and an AC component which is removed by filtering. The DC component is integrated until the integrated value attains a threshold, at which time a change of state in the output of the metering device indicates that the consumption of a predetermined quantum of electric energy has occurred.
The multiplication of the two analog variables is achieved by alternately switching the polarity of one of the analog variables in response to a control signal which is pulse-width modulated by the other analog variable. The result is a pulse train whose height is responsive to one of the variables and whose instantaneous pulse width is responsive to the other analog variable. Accordingly, the average or DC component of the resultant width and amplitude modulated pulse train is proportional to power consumption.
The pulse-width modulator in each of the foregoing patents employs a comparator for measuring the time at which the voltage signal passes a predetermined reference voltage. Thus, the accuracy with which the pulse width is controlled is dependent upon the accuracy of the reference voltage. In addition, each of these prior devices requires that at least one of the analog signals be a balanced, double-ended signal. A balanced, double-ended signal is conveniently generated using a center-tapped transformer. Such a center-tapped transformer is expensive and its elimination is thus desirable. Furthermore, it is desirable to reduce the amount of power consumed by the metering equipment. This indicates that solid state devices, and preferably low-power-consumption, solid state devices should be used. One desirable type of solid state device presently available is a complimentary metal oxide semiconductor (CMOS). The circuits of the above-referenced patents are not efficiently implemented in CMOS technology.